1. Field of the Invention:
The present invention relates to a blast-furnace tuyere having excellent thermal-shock resistance and high durability.
2. Description of the Prior Art:
In general, the tuyere composed of copper or copper alloy as a substrate is exclusively used in a water-cooling fashion for blowing hot air into a blast-furnace. However, since the end portion of the tuyere projects into the blast-furnace and is exposed to a severe environment in the furnace, it is particularly liable to be damaged due to the overheating by contacting with molten iron or slag. Consequently, an explosion accident may be caused by leaking out water used for cooling the tuyere and also a heat loss and a considerable reduction of tapping amount are brought about by lowering the temperature inside the furnace. Furthermore, the exchange of the damaged tuyere is a dangerous operation and it requires a great amount of labor and time. In a recent large-scale blast-furnace, the temperature of hot air blowing into the furnace is above 1,300.degree.C and further blowing of heavy oil or oxygen and high-handed operation are adopted, so that the condition of use of the tuyere becomes more severe. Therefore, it becomes more important to develop technics for preventing the damages of tuyeres per a blast-furnace as the blast-furnace becomes large-size.
Heretofore, various attempts have been made to the use of the tuyere obtained by applying a metal coating 2 to a copper substrate 1 of the tuyere body and applying a ceramic coating 3 to the metal coating 2 as shown in FIG. 1. As a successful example of these attempts, there is known a tuyere composed of a copper substrate, a metal coating consisting of 60-62% of nickel, 12-15% of chromium and the remainder of iron, manganese and carbon, and a ceramic coating of molten alumina (Al.sub.2 O.sub.3) wherein the thickness of the metal coating is 0.0127-0.508 mm, preferably 0.0508-0.1778 mm and that of the ceramic coating is 0.0254-1.016 mm, preferably 0.127-0.381 mm. In this example, the metal suitable for the use as the metal coating includes austenitic steels of AISI standard 301, 302, 302B, 303, 304, 308, 309, 310, 316, 321, 347, etc., chromium steels of AISI standard 403, 405, 406, 410, 414, 420, 430, 431, 440A, 440B, 440C, 442, 443, 446, 501, 502, etc., and pure nickel. However, these metals have not a chemical affinity to the copper substrate but are mechanically bonded to the substrate, so that they are apt to peel off from the substrate and are not particularly suitable.
Further, in order to apply the ceramic coating to said metal coating, it is known to use ceramics such as alumina, beryllium oxide, calcium oxide, cerium oxide, chromium oxide, chromite, magnesia, silica, strontium oxide, zirconia, zirconium oxide silicate and the like.
Moreover, in the prior art, the expansion coefficient of the metal coating (e.g., expansion coefficient of the above mentioned alloy: about 14-15.times.10.sup.-.sup.6) is defined to be intermediate between expansion coefficients of the copper substrate (expansion coefficient of pure copper: 16.5.times.10.sup.-.sup.6) and the ceramic coating (expansion coefficient of the ceramic: about 7.5-9.0.times.10.sup.-.sup.6). However, the difference of expansion coefficient between the metal coating and the ceramic coating is considerably large in practice. Therefore, in the practical use of such a tuyere, the ceramic coating peels off from the metal coating at the deposited surface, so that the operation time of the said tuyere is not substantially prolonged as compared with that of a tuyere composed only of copper substrate.